Since electrochemical parts utilizing solid electrolyte have no problem of solution leakage, can be formed into films or thin membranes and can also be miniaturized, they can be easily applied to portable electronic goods or motorcars in comparison with usual electrochemical parts using liquid electrolyte.
Particularly, a electrochemical cell using a solid polymeric ionic conductive membrane is light performs charging and discharging in high efficiency and can be adapted to various shapes. Accordingly, such polymeric ion conductive membrane has been the focus of researches and developments.
Recently, a process for preparing a electrochemical cell wherein a polymeric compound is used as electrolyte has been developed. Since the polymeric compound can be formed into a thin membrane, and has the ability to dissolve salt and transport ions it can be used as an electrolyte. The resulted solid electrolyte has small electric resistance and large electric current.
As set forth hereinabove, the electrochemical cell of said polymeric electrolyte should preserve both high ionic conductivity and good mechanical feature. However, it is difficult to improve both properties by merely changing mean molecular weight or the glass transition temperature of its polymer matrix.
Consequently, there is a long-felt need to develop a novel polymeric electrolyte which has both improved ionic conductivity and mechanical feature.
U.S. Pat. No. 4,654,279 to Bauer et al. disclosed a solid polymeric electrolyte which is two phase interpenetrating network consisting of a mechanically supporting phase of a continuous network of a cross linked polymer to improve the mechanical feature of the solid electrolyte and an ionic conducting phase which provides an ionic transmission path through a matrix.
U.S. Pat. No. 4,556,614 to Le Mehaute et al. disclosed a method of producing a solid electrolyte for an electrochemical cell, the electrolyte comprising at least one complexing polymer and at least on ionizable alkaline salt complexed in the polymer, wherein said complexing polymer is mixed with at least one second polymer miscible with the complexing polymer and having cross-linkable functions; said cross-linkable functions then being physically cross-linked, with the complexing polymer being brought to an essentially amorphous state during said cross-linking operation.
Xia et al. studied on the heat and ionic conductivity of a polymeric ion electrolyte prepared by polymerization of oligo-oxymethyl methacrylate. (Solid State Ionics. 1984, 14, p.221-224).
Also Korean Laid Open Patent Application No. 95-8604 to Kwang-Kyong Rhew disclosed a composition for an UV rays curing type-lithium ionic electrolyte of polymeric solid which comprises an UV rays curing type lithium salt, an acrylate and/or methacrylate of polyalkylene glycol having at least one unsaturated reactive radicals, an UV rays curing initiator and a curing promotor, and a method for the preparation thereof. However the composition can hardly be formed into an exact amorphous film (thin membrane) in practice. Thus the ionic conductivity of the resulted film changes remarkably as the temperature becomes different. Further the ionic conductivity at room temperature decreases significantly as time goes by.
In addition, since the back bone of the polymer comprises merely oxymethyl methacrylate unit, the resulted polymeric membrane is too hard, the ionic conductivity is not so high and the electrolyte membrane is fragile. Accordingly, it was difficult to apply the polymeric membrane to electrochemical solid parts.
The polymeric electrolyte prepared by the above process has a low ionic conductivity at room temperature (below 1.times.10.sup.-5 S/cm), and a weak adhesiveness to the substrate.
When applied as a solid electrochemical parts, the polymeric electrolyte is cracked and its durability is reduced. Electrochemical parts or apparatus utilizing the polymeric electrolyte thus have reduced durability.
In order to solve the above problems, the inventors of the present invention started a research on polymeric ion electrolyte which can be prepared by polymerization of an oxyethyl methacrylate, a cross linking agent and a plasticizer.
The inventors have found, as a result of the research, when units derived from adequate cross linking agent and plasticizer are introduced into the chain of the polymer; the electric conductivity of the resulted polymeric ion electrolyte at room temperature increases to more than 10.sup.-4 S/cm and the mechanical feature improves. Further, the said polymeric ion electrolyte can be formed in thin films and still has an excellent adhesiveness.
In addition, the solid polymeric ion electrolyte membrane so produced can be in the form of a free standing film, which has an excellent adhesiveness when adhered to the substrate again.
The inventors have recognized that the said polymeric ion electrolyte membrane according to the present invention is neither cracked nor does it form pores even after more than three thousand cycles of charging and discharging which were carried out under a voltages from -3V to +3V, and that the membrane maintained excellent adhesiveness and can be applied to solid electrochemical parts.